Method of preparing uranium trioxide



Patented Aug. 2, 1949 ,METHOD OF PREPARING URANIUM TRIOXIDE Sherman M.Fried, Chicago, 111., and Norman B. Davidson, Sierra Madre, Calif.,assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission No Drawing. Application January 15,1948, Serial No. 2,558

4 Claims.

This invention relates to the preparation of oxides of uranium incrystalline form, and more particularly relates to the preparation ofhigher oxides of uranium from lower oxides of uranium.

heating at temperatures between 450-750" C. in

the presence of oxygen under a pressure varying from 20-150 atmospheres.

In a further embodiment of this invention a The crystalline oxides ofuranium and espe- 5 lower oxide of uranium, such as uranyl uranatecially the crystalline trioxide of uranium are (U308) in eitheramorphous or crystalline form compounds particularly well suited toprocesses is converted to the higher oxide (U03) in crystal- Ior theproduction of uranium pentachloride. line form by heating within thesame tempera- Since preparation of uranium trioxide by a procture rangeand subjecting to oxygen under the ess such as the thermal decompositionof hydratsame pressure conditions as previously set forth ed uranylperoxide usually produces uranium triherein. It is apparent from thedata listed in oxide in an amorphous form, a process for the the tablethat the time required for conversion of production of crystallineuranium trioxide is paruranyl uranate (U308) to the higher oxide (U03)ticularly advantageous. varies from 12-112 hoursdepending upon the It isthus an object of this invention to pro- 15 pressures and temperaturesto which the system vide a method whereby an amorphous oxide of issubjected. uranium can be converted to crystalline form. In a furtherembodiment of this invention the It is a further object of thisinvention to procrys a i e m of uranium Oxide is p p ed videa methodwhereby a lower oxide of uranium more rapidly y treating an amorphousuranium can be converted to a higher oxide. oxide with oxygen undersuperatmospheric pres- A further object of this invention is to providesures ranging between 60-150 atmospheres at a a method whereby a loweroxide of uranium i temperature from YOU-750 C. for periods as short theamorphous phase can be converted to a as 1 /zhours- It is apparent fromthe data set higher oxide in the crystalline form. forth hereafterthatthe temperatures and pres- Other objects and advantages willbecomeap- 5 sures necessary for conversion of the amorphous parent uponfurther examination of this specifi- Oxide and e lower o de to thehigher oxide in cation. crystalline form vary directly with each otherIn accordance with the process of this invenand inversely w t t du at nof a nt 01 tion an amorphous oxide of uranium is convertthe startingmaterial. This is further illustrated ed to uranium oxide in crystallineform by subby the reaction wherein uranyl uranate (U308) is jecting theamorphous uranium oxide to superatconverted to the hexagonal uraniumtrioxide mospheric pressures of oxygen at an elevated (U 3 in'100% yieldby treatment e ther t e temperature. The amorphous oxide of uraniumamorphous or crystalline uranyl uranate (U308) is heated at temperaturesbetween 450-750 C. with oxygen under a pressure from '60-150 atunderoxygen pressures varying from 20-150 atmospheres at a temperature from700-750" C. for

mospheres. The crystalline product thus formed periods as short as 1/2-2 hours. is characterized by its color as well as the crystal Thetemperature and pressure, conditions used lattice constants andcrystalline form. in several experiments, wherein amorphous ox- In oneembodiment of this invention amorides of uranium are converted tocrystalline phous uranium trioxide (U03) is converted to 40 uraniumtrioxide, are set forth in the following the crystalline form of thesame compound by table.

1 Formula of Formula of Substance Temp., Oxygen Time Phases Present inprod: product Calcd product Cal ed treated 0. Pressure uct (by X-rayAnalysis) from Oxygen from Ignition Uptake to U308 Aim. Hrs.

28 12 hexag. 28 86' -hexag.+phaseII 28 36 --d0 -rs.-- U02. 30 36 do1703.005 30 112 phase II+pheselII U0m1 Omn 30 112 hexag.+phase TT 150 2phase TTT H L5" -do. .n. 1702.99: 1703.01

8 The U0; was prepared by heating UOLZHQO to 400 C.

UO; actually lost 0.254% in weight under these conditions, probablybecause of moisture or oxides of nitrogen present in the originalcompound. If this weight loss is assumedtobedue to moisture, thecalculated compositionoi the starting material was UO3.O.4HzO. 1

'lhe packing density of this material was measured as 3.88.

in the 7 structure.

In the above table, the substance referred to as phase II is describedas having a red color which is identified as a hydrated form of uraniumtrioxide and is less stable than that phase identified as phase III,which is a yellow crystalline material having the same latticedimensions and hexagonal crystal structure as set forth herein foranhydrous uranium trioxide.

The apparatus for heating uranium oxides under superatmosphericpressures Qof oxygen consists of a heavy-walled glass tube (I. D. 4 mm.,7

0. D. 12 mm.) to which a short length of 1 mm. capillary is sealed. Thetube is attached to a vacuum apparatus and pure dry oxygen is condensedin the small capillary to a measured length which determines thepressure in the bomb when it is sealed off at the proper point. Thepressures quoted in the various experiments were calculated from themeasured length of liquid oxygen, assuming that the gas obeyed theperfect gas laws even at high pressures.

Asa...

safety precaution the glass bomb was placed inside a steel jacket fittedwith'plugs and the entire assembly was heated in a tube furnace.

' Annealed Pyrex bombs could usually withstand pressures of 30atmospheres at 500C. For experiments at higher pressures andtemperatures quartz bombs were used. A quartz bomb of 3 mm.

I. D. and 10 mm. 0. D. withstood '70 atmospheres oxygen pressure at'750- 0., while a quartz capillary bomb 1 mm.'I D'Jand' 10 mm. 0. D.withstood a pres-sure of 150 atmospheres at 700 C. The identification ofthe uranium trioxide produced by the process of this invention is basedonthe'observed color and the X-ray diffraction pattern.

"Uranium trioxide is hexagonal' The unit cell contains one molecule andhas thelattice dimensions: 7 t p 1,

V a1=3.963i.004 A. (13:4.1601008 A;

V The calculated density is p=il.34.

Thespace group is 03m and the atomic positions arez l U in 0 O), 1.01 in(0 0 V 2011 in /3 u) with u-0.17. .7 .Each' uranium atom is bonded totwo oxygens of the first kind with U-01=2.08 A. and to six oxygens ofthe second kind with U0n=2.39 A. 1 The or-atoms should be regarded asuranyl oxygens, although there are no (UOz)+ groups Instead, there areendless uranyl chains -O1-U-,-O1-'U-O1U along the (13 axis. j

The uranyl uranate is orthorhombic with lattice dimensions as follows:

and two uranium atoms 'per cell at (0 0 0) 0/ /2 0). Since there are 5%;oxygens in thisv cell and weak lines were observed in long expos urepatterns, we have concluded that the true unit cell of uranyl uranate ismore accurately represented by atripling of the ca axis. The true unitcell thus contains six uranium atoms and 16 oxygen atoms, and thelattice constants are as follows: 7

4 Referred to the larger cell, the positions of the uranium atomsbecome:

2Uin(000) /2 /20) 4Uini- 0 V30: /2 A30) It has been observed that thereis a strong similarity between the X-ray diffraction pattern obtainedfor the crystalline uranyl uranate, U308, and the crystalline trioxide.

The Usoa-structure is obtained from the U0:- structure by means ofslight changes. This is shown by referring the UOa-structure toorthohexagonal axes and choosing the 0.2 axis three times longer thanrequired. The cell so obtained has dimensions:

and the coordinates for the uranium atoms are exactly the same as givenfor U303 above. The oxygen positions in the two structures must be verysimilar also, the important diiference being that there are no 'uranyloxygens at (0 0 /2) /2 /2 /z) in the U308 structure so that the uraniumatoms at (0 0 0) (/2 Va 0) are to be regarded as U+ atoms.

The examples following are illustrative for the process of thisinvention but are not to be construed as'a limitation upon the appendedclaims.

EXAMPLE I 'those listed previously for crystalline uranium trioxide(U03).

EXAMPLE II 7 'A sample of uranyl uranate that had been ignited in air at800 C. was treated with oxygen 'at a pressure of 28 atmospheres for 36hours at 500 C. The color of the solid changed from 7 black to deep redand X-ray analysis of the product indicated the presence of two phases:namely hexagonal uranium trioxide (phase I) which is isomorphous withthe form obtained in Example I, and a new phase (phase II), which iscolored red. The formula of the hexagonal product formed (phase I), ascalculated from the weight difference measured with reference to theuptake of oxygen, is UOzm.

A second sample of uranyl uranate (U308) was heated for a total of 112hours at a temperature between 530 and 560 C. with oxygen at a pressureof 30 atmospheres. The product consisted of two phases visible to thenaked eye which could be separated mechanically. One phase was coloredyellow and gave a pattern identical with that of uranium trioxideprepared by the ignition of the hydrated uranyl nitrate,

' (UOzNOaGHzO).

The formula of the product, as calculated from the oxygen uptake, isUOaw.

' A 0.07267 g. sample of this product was ignited at 900C. and thusconverted to uranyl uranate (0.07136 g.). The formula of the productformed by the oxidation of uranyl uranate was calculated from the weightloss (0.00131 g. equivalent to 1.804%) on ignition, to be UOazasr.

A sample of uranyl uranate heated at 700 C. for two hours with oxygen ata pressure of 150 atmospheres produced uranium trioxide in the yellowcrystalline phase (III).

EXAMPLE III Uranyl uranate heated to 750 C. for one and one-half hoursin the presence of 62-70 atmospheres of oxygen likewise formed theyellow crystalline phase, whose formula, as calculated from the oxygenuptake, was determined as UO2.993 and, as calculated by the ignition touranyl uranate, was UO3.01. In all cases where uranyl uranate is ignitedin oxygen at high pressures to form uranium trioxide the crystalstructure of the product is that of anhydrous uranium trioxide ratherthan that of a hydrate.

The yellow crystalline phase of uranium trioxide (phase III) is stablefor several hours in the presence of 1 atmosphere oxygen at 700 C. butat 800 C. is decomposed to uranyl uranate in contrast to the amorphousuranium trioxide which decomposes to UO2.959 at 620 C. and to UO2.s2 at625-630 C.

The X-ray difiraction patterns for UO2.959 and UOasz are both identicalhaving the following lattic dimensions:

a1=6.90:0.02 A. a2=3.91i0.02 A. a3=4.15i0.02 A.

EXAMPLE IV Ignition of (hydrated) uranium trioxide A brick red coloredsample of uranium trioxide having a bulk density of 4.5 was heated at600 C. for one-half hour with oxygen at one atmosphere pressure. Thesample decreased in weight by 0.92% and microscopic examination revealedthe presence of two phases, one yellow and the other dark green. Thephases could be partially separated by mechanical method and wererespectively identified by X-ray analysis as uranium trioxide and uranyluranate, the former being isomorphous with the hexagonal uraniumtrioxide. On heating the brick red sample in oxygen at 1 atmospherepressure at 650-675 C. the weight loss was 0.06% and there was no changein appearance. Upon ignition in air at 850 (3., the weight loss was1.67% and the procl not was uranyl uranate. The sample consisted of 87%of the anhydrous yellow uranium trioxide after treatment at 650-675 C.

The foregoing descriptions comprise the preferred embodiments of thisinvention. However, many alternations and changes may be made thereinwithout departing from the spirit and scope of this invention as setforth in the appended claims, which are intended to cover as broadly aspossible in view of the prior art, all features of novelty taken singlyand/or in combination.

What is claimed is:

1. A process for the preparation of uranium trioxide in crystallineform, comprising heating an amorphous oxide of uranium at a temperaturebetween 450-750 C. in the presence of between 20-150 atmospherespressure of oxygen.

2. A process for the preparation of a crystalline uranium oxide,comprising heating uranyl uranate at a temperature between 450-750 C. incontact with oxygen, said oxygen being present at pressures between20-150 atmospheres.

3. A process for the preparation of a crystalline uranium oxide,comprising heating uranyl uranate at a temperature between 700-750 C. incontact with oxygen, said oxygen being under a pressure between -450atmospheres.

4. A process for the preparation of crystalline uranium trioxide,comprising heating amorphous uranium trioxide at a temperature between450-750 C. in contact with between 2 atmospheres pressure of oxygen.

SHERMAN M. FRIED. NORMAN R. DAVIDSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

Hopkins, Chapters in the Chemistry of the Less Familiar Elements, vol.II, chap. 18, Uranium, pages 11 and 12. Pub. in 1940 by StipesPublishing Co., Chicago, Illinois.

